Ferrofluids are a type of magnetic fluid that typically consist of colloidal magnetic particles such as magnetite or manganese-zinc ferrites, dispersed with the aid of surfactants in a continuous carrier phase. The average diameter of the dispersed magnetic particles ranges between 5-10 nm. Each particle has a constant magnetic dipole moment proportional to its size that can align with an external magnetic field.
Ferrofluids experience body forces in homogeneous magnetic fields, that allow their position to be manipulated, and thus enable the construction of devices such as rotary seals, bearings, and related mechanical devices. Ferrofluids also have been used to construct display devices such as those disclosed in U.S. Pat. Nos. 3,648,269 and 3,972,595, that use a magnetic field to capture an opaque magnetic fluid in a predetermined optical pattern. These types of devices usually operate by having an opaque magnetic fluid displace a transparent fluid and thereby produce optical contrast. Such display devices, however, do not generate ordered crystalline structures in the magnetic fluid, and are incapable of generating anything other than a monochromatic image.
Two general methods for producing ferrofluids have been used in the prior art. The first method reduces a magnetic powder to a colloidal particle size by ball-mill grinding in the presence of a liquid carrier and a grinding aid which also serves as a dispersing agent. This approach is exemplified in U.S. Pat. Nos. 3,215,572 and 3,917,538. The second approach is a chemical precipitation technique as exemplified in U.S. Pat. No. 4,019,994. Both of these techniques suffer from the disadvantage that there is heterogeneity in the size distribution of the resulting magnetic particles, the composition of these particles, and/or the interaction forces between the particles. This heterogeneity may produce deleterious effects on the ability of a ferrofluid to form ordered structures under the influence of a magnetic field.
Pattern forming systems of magnetic fluid films under the influence of external magnetic fields have recently attracted much interest. For these studies, a variety of different types of magnetic fluids have been used. For example, the aggregation process and one-dimensional patterns formed in suspensions of latex or polystyrene particles loaded with iron oxide grains under the influence of parallel fields have been studied by M. Fermigier and A. P. Gast, J. Colloidal Interface Sci. 154, 522 (1992), and D. Wirtz and M. Fermigier, Phys. Rev. Lett. 72, 2294 (1994). Quasi two dimensional periodic lattices have been reported to be formed in a phase separated magnetic fluid thin film under the influence of a perpendicular magnetic field. Wang et al., Phys. Rev. Lett. 72, 1929 (1994). FIG. 1 of this paper, however, shows that the resulting structure is disordered. Other investigators have generated more highly ordered two dimensional lattices in thin films of magnetic fluid emulsions or magnetic fluids containing non-magnetic spheres using perpendicular magnetic fields. However, these lattices tend to solidify and therefore are not suitable for applications requiring rapid interconversion between crystalline and amorphous states. See, e.g., Liu et al., Phys. Rev. Lett. 74, 2828 (1995), Skjeltorp, Phys. Rev. Lett. 51, 2306 (1983). Thus there is a recognized need in the art for ferrofluidic compositions that could be used to generate liquid-crystal devices that could be switched by small magnetic fields. See, e.g., da Silva and Neto, Phys. Rev. E. 48, 4483 (1993).
If a ferrofluid composition capable of reversibly forming ordered one dimensional structures or crystalline two dimensional lattices in a thin film under the influence of an external magnetic field could be manufactured, it would be useful for constructing a variety of new and useful liquid-crystal magneto-optical devices. For these reasons, a method is needed for generating homogeneous ferrofluidic compositions capable of reversibly forming ordered one dimensional structures or crystalline two dimensional lattices in a thin film under the influence of an external magnetic field. Also needed is a simple method for determining whether a thin film of a ferrofluidic composition is capable of generating well-ordered one dimensional structures or two dimensional lattices under the influence of external magnetic fields. Finally, it would be desirable to generate magneto-optical devices based on the ordered structures created in thin films of ferrofluidic compositions in response to external magnetic fields. Because the utility of such devices would be enhanced by developing methods for controlling the ordered structures formed in magnetic thin films of ferrofluids under the influence of external magnetic fields, methods for controlling the ordered structures so formed also are needed.